Abstract: The fracture toughness of the weld joints of aluminium alloy in the argon and helium arc welding was Investigated by means of surface crack testing.It is known from the testing,that the fracture toughness of weld metal and the overheating zone are the lowest and fracture toughness of HAZ decrease in a different extent,accoding to the welding parameter i.e.the heat input.By means of metallography and fractography analysis,the cast structral characteristic of the weld metal without deformation processing leads to get nothing in the heat treated strengthening effect as in the deformed Al-alloy.The fusion and solidification of the eutectic in the weld overheating zone during the heating cycle is concerned along the grain boundary.It is the critical effect for the decrease of the fracture toughness to a lowest value in this overheating zone. And the annealing effect induced to the HAZ in a different degree,depend upon the actuating time-temperature in welding.